The present invention relates to the measurement of flow rates of gas/water vapor mixtures through conduit and more particularly to the measurement of the gas component and the water vapor component under vacuum flow conditions in the conduit.
The measurement of steam and water flow rates in steam power plants (both conventional and nuclear) is a required operation at a variety of junctures in the various steam and water circuits comprising the steam power plant. Referring to FIG. 1, steam produced within a boiler vessel 10 is taken through steam line 12 and applied to steam turbine 14. Exhaust steam 16 from turbine 14 is condensed in a condenser 18 and returned via line 20 and pump 22 to pressure vessel 10 via line 24 as feedwater. Cooling water is passed through a multiplicity of tube, such as tube 17, connected between headers 19 of condenser 18 via line 26 by pump 28. Finally, vacuum pump 30 maintains reduced pressure in condenser 18 via line 32 with line 34 vented to atmosphere. It is desirable to measure the flow rate of air that enters condenser 18 via leaks to this low pressure region and removed by pump 30, with flow sensor 36 located in line 32.
Over the years, industry has recognized the importance to minimize the back-pressure at the exhaust of the steam turbines where such low pressure condensers are employed. As exhaust pressure rises, more fuel is required to generate additional steam to maintain turbine output power. An increase in the back-pressure decreases plant operating efficiency. In the electrical power industry, millions of dollars per year per generating plant can be lost due to unnecessary rise in turbine exhaust pressure. A manageable cause for increased back-pressure is air leakage into the evacuated end of the turbine and its exhaust steam condenser.
To remove air and other non-condensable gases that enter the condenser space, a vacuum pump, air injector, or some other contrivance is employed. For systems most commonly employed, a line or conduit generally is installed between the condenser and the pump within which these gases are conveyed. They enter the air removal equipment generally at a pressure in the range of about 1 to 6 inches of mercury absolute where they are compressed to atmospheric pressure and released to the environment.
In addition to non-condensable gases passing through the conduit, an amount of water vapor is contained which generally is determined by the absolute pressure within the condenser and the temperature of cooling water passing through the condenser tubes. The partial pressure of the water vapor generally is related to the temperature of the cooling water. This pressure represents the minimum obtainable back pressure available to the turbine and generally is related to local environmental conditions. Pressures higher than this which are caused by air leaks require attention because this added pressure component can be diminished by maintenance procedures.
Flow instruments available on the market have limitations which preclude their use for measurement of air flow in the conduit at low pressure. The mass flow rate is below the low end sensitivity of most instruments. Further, the variable concentration of combined air and water vapor flow cannot properly be separated such that air only is measured.
The most common method of measuring air leaks is to physically observe the height of a float in a variable area flow meter or the pressure drop across an orifice in a differential pressure flow meter, either of which is connected to the exhaust end of the vacuum pump. In addition to being inconvenient, high inaccuracy is present using these methodologies. The air is contaminated with liquid water due, in part, to condensation of the pumped water vapor from the condenser being raised to atmospheric pressure and also from pump designs which use a water seal, both of which adds buoyancy to the float or partial closure of the orifice. Another limitation of these methodologies is that the measurement requires plant personnel to observe and record at intervals which is inconvenient and costly to the plant operator. Further, without an output signal that can be recorded in real time, the measurement cannot be correlated in time with other events which could be used to identify sources of leaks. Also, because of the measurement location, the measurement cannot be used to determine if the air flow results from a fault pump seal or an air leak into the condenser space.